linux-kernel - Re: [PATCH v3] perf/core: Set event shadow time for inactive events too

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Date:   Mon, 20 Dec 2021 10:30:34 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     Namhyung Kim <namhyung@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>
Subject: Re: [PATCH v3] perf/core: Set event shadow time for inactive events
 too

On Sat, Dec 18, 2021 at 09:09:05AM +0000, Song Liu wrote:
> 
> 
> > On Dec 17, 2021, at 8:35 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > On Fri, Dec 10, 2021 at 11:33:41AM +0100, Peter Zijlstra wrote:
> > 
> >> I'm thinking this is a cgroup specific thing. Normally the shadow_time
> >> thing is simply a relative displacement between event-time and the
> >> global clock. That displacement never changes, except when you do
> >> IOC_DISABLE/IOC_ENABLE.
> >> 
> >> However, for cgroup things are different, since the cgroup events aren't
> >> unconditionally runnable, that is, the enabled time should only count
> >> when the cgroup is active, right?
> >> 
> >> So perhaps perf_event_read_local() should use a cgroup clock instead of
> >> perf_clock() for cgroup events.
> >> 
> >> Let me think about that some more...
> > 
> > How's this then? Song, could you also please test and or better explain
> > the problem f79256532682 pretends to cure? Because the below is
> > reverting that, I *really* hate having to touch the events we're not
> > scheduling.
> 
> Unfortunately, this change bring the bug back. For time_enabled in rdpmc
> case to work properly, we have to touch all the enabled but not running 
> events, right?

Ohh.. argh. I think I see why, it looses the context time enable edge,
and because this is all strictly per-event in the uapi (there is no ctx
representation) it can't be cured by improving ctx time handling :/

Bah, I so hate this.